A man accused of murdering his former girlfriend told a prison inmate he had "put her to sleep", a court has heard.

The inmate was given name suppression when he gave evidence before Justice Miller and a Christchurch High Court jury on the third day of Nikki Roper's trial on a charge of murdering Alexsis Maria Tovizi in Christchurch in December 2010.

Roper denies that charge, as well as unlawfully taking her car, stealing her laptop computer and five charges of dishonestly using her bankcard.

The prisoner said today that he had known Roper for about five years.

In a conversation before Christmas 2010, they spoke about a tattoo on Roper's cheek that Roper said was the name of his girlfriend. The tattoo looked fresh because the lettering was dark.

Roper made a comment about "putting his girlfriend to sleep", and the man asked him what he meant.

"He said, 'I killed her; she's a slut'," the man said.

Roper appeared to be laid back as though he did not have a care in the world. He had behaved as though it was some sort of joke, the man said.

Another inmate, who also has name suppression, told of being in a cell at Christchurch Men's Prison with another man in February 2012.

While they were in their cell, another person came to the door and asked the cellmate what had happened at court the previous day.

When the cellmate said he had pleaded guilty, the man at the door said he was dumb to plead guilty. The man at the door then said: "I pleaded not guilty and I killed my missus."

The defence accepted that the man at the door was Roper.

A neighbour of Tovizi in Stanmore Rd, Linwood, told of hearing an argument between her and Roper outside their unit one or two days before her death.

He saw Roper reach through the open window of Tovizi's car and take the car keys.

In earlier evidence, a friend of Roper's told of him staying at her property after his release from prison on December 1, 2010.

She told of an argument between him and Tovizi, which included mention of her engagement to another witness in the trial, Daniel Ross Edwards.

In a later message from Roper, he told the the witness that Tovizi had picked Edwards over him and they were "over for good". The witness has name suppression.